The FCC Process has been a major petroleum refinery unit facility for about forty years in the capacity of converting petroleum fractions heavier than gasoline, boiling above about 400.degree. F., into high octane naphtha suitable for blending as a major stock in the manufacture of motor gasoline. Typically, preheated petroleum fractions in the nature of gas oils and heavier (boiling ranges above about 550.degree. F.) are contacted with hot cracking catalyst of a size suited to fluidization, say 200 mesh, under conditions to suspend or fluidize the powdered catalyst in vapor of the charge. Conversion of the charge takes place at the contact temperature in excess of 850.degree. F., usually 950.degree. F. or higher, up to about 1000.degree. F. In general, the major product sought in naphtha suitable for use in motor gasoline having a boiling range upwards of about 100.degree. F. to 375.degree.-425.degree. F. This is accomplished by cracking of the charge components to lower boiling compounds in the motor fuel range.
The cracking reaction is accompanied by a number of other reactions such as polymerization, hydrogen exchange, isomerization and the like. In addition, primary products of cracking are susceptible to further cracking and other reactions. The net result of this complex of reaction paths is endothermic overall, that is, the cracking conversion consumes heat in an adiabatic system resulting in a drop in temperature of the mass of reactants and catalyst. The heat required to bring the mass to reaction temperature and to satisfy the endothermic heat of reaction is derived solely from sensible heat of the charge stock and catalyst. Since it is undesirable that the charge undergo thermal cracking which yields much lower octane number naphthas, preheat of the charge is generally limited to about 700.degree. F. or lower, leaving the major burden of heat supply to be borne by the catalyst.
Among the reaction products in addition the desired naphtha are gas oils, kerosenes, light hydrocarbons of one to four carbon atoms and a carbonaceous deposit on the catalyst surfaces (commonly called "coke") which masks the active sites of the catalyst surfaces and renders the same inactive because unable to make contact with the molecules of the charge and induce reaction. The coke is removed by burning in air to regenerate activity of the catalyst in a vessel to which the inactivated (spent) catalyst is transferred from the reactor. The catalyst is heated by the burning of coke, thus reaching an elevated temperature at which it is returned to the reactor for supply of heat to bring charge to reaction temperature and to supply endothermic heat of reaction.
Modern FCC units operate in a heat balanced mode in which the amount of catalyst returned to the contact with charge in the reactor is automatically controlled to maintain a desired reaction temperature. Thus an increase in regenerator temperature automatically results in reduced catalyst flow from regenerator to reactor as the instruments detect a tendency for increased reactor temperature. Thereby an important reaction parameter is necessarily affected by the reduction in catalystto-oil ratio (cat to oil or C/O) which corresponds generally to the space velocity parameter of fixed bed catalysis. The reduction in C/O reduces severity of the conversion, as increased space velocity reduces severity in fixed bed reactors. This absolute interdependence of variables is a major characteristic of FCC commercial units and has great significance in operation according to this invention.
The advent of zeolite cracking catalysts in the early 1960's resulted in an important shift in the nature of catalytic cracking in general and FCC in particular. See, for example, U.S. Pat. No. 3,140,249. These cracking catalysts yield significantly less coke and dry gas than do the older catalysts of amorphous silica-alumina at the same level of conversion and are much more active in that they induce a higher level of conversion measured as yield of products outside the boiling range of the charge at the same conditions of reaction. It will be seen that the zeolite catalysts provide less "fuel" to be burned in the regenerator for supply of heat required by the reactor.
The course of reaction in the regenerator involves oxidation of the coke, with the small amount of hydrogen in the coke being converted to water. The primary reaction products of oxidizing carbon are carbon monoxide and carbon dioxide. The latter represents complete oxidation of carbon, extracting the fullest measure of heat generation from the fuel. The carbon monoxide content of the gases derived from regeneration constitutes a potential fuel and is regarded as a contaminant if present in the flue gases discharged to the atmosphere. It has been conventional practice to pass the flue gases from FCC regenerators to boilers for combustion of carbon monoxide and recovery as steam of the heat energy derived from that combustion as well as that available from ensible heat of the flue gas. Such "CO boilers" must maintain a temperature high enough to promote combustion of CO, about 1500.degree. F. To maintain that temperature, it is customary to supply supplemental fuel (gas or heavy liquid) to the CO boiler together with the quantity of air required for combustion of CO and supplemental fuel.
As is well known in this art, there is a tendency for burning of carbon monoxide in the FCC regenerator, a type of operation which has, in the past, been a source of problems and has been suppressed by limiting the air supply to the regenerator with consequent damping of the coke burning and by injection of water or steam to the space above the dense fluedized bed in the regenerator in order to quench burning of carbon monoxide. As the gases from combustion of coke rise from the dense fluidized bed in which burning regeneration is conducted, they enter a spaced above the dense bed. The gases so disengaged from the dense bed carry with them a small amount of entrained catalyst and constitute a "disperse phase" of minor amounts of catalyst in a rising mass of gas which contains carbon dioxide, carbon monoxide and unconsumed oxygen as well as water vapor, nitrogen, etc. This combustible mixture can and does undergo partial reaction of carbon monoxide and oxygen with release of large amounts of heat in the disperse phase. Since the amount of catalyst in this disperse phase is small, the heat is diverted to heating of the flue gas and temperature of that mass rises rapidly. The adverse effects of excessive temperatures at this stage by irreversible deactivation of catalyst and damage to regenerator internals by exceeding metallurgical limits are so great that extensive and ingenious expedients have been considered as control means. The most widely adopted until quite recent times has been introduction of quench media, water, steam, etc., to the disperse phase or within such regenerator internals as cyclone separators, plenum and the like.
More recently, developments have been made which permit burning of CO in the regenerator by constraining that burning to a region of relatively high catalyst density such that the heat of CO combustion is largely absorbed in heating of particles of solid catalyst. One of those techniques manages to cause catalytic burning of CO in the dense bed where catalyst density is high under conventional conditions of operation. Another technique permits conventional thermal burning of CO in the disperse phase and injects thereto large amounts of catalyst to increase catalyst density of the disperse phase greatly above that encountered in conventional operation. The first mentioned technique of moving the combustion reaction to a region of conventionally high catalyst density is described in British Pat. No. 1,481,563 published Aug. 3, 1977. The other technique of moving catalyst to a region of conventional CO combustion is described in U.S. Pat. No. 3,909,392, dated Sept. 30, 1975. The full disclosure of both cited patents on CO combustion are hereby incorporated herein by this reference.
By any technique of burning CO in the regenerator in the presence of large amounts of catalyst, it becomes possible to raise the temperature of regeneration thus raising the rate of coke burning to provide regenerated catalyst of lower residual coke content and hence more active. These techniques also permit recovery of a greater proportion of the fuel value of the coke within the FCC cycle of reactor and regenerator for direct use in heat balancing the unit. As would be expected, the CO burning techniques require increased supply of air to assure an excess of oxygen for complete or partial combustion of CO as desired. In general, these techniques result in higher temperature of regenerated catalyst and necessarily cause reduction of the cat/oil ratio, well compensated by the higher activity of the cleaner (less coke on regenerated catalyst) catalyst so produced.
A further important advance in FCC technology is the so-called "riser reactor" in which hot catalyst and charge stock are supplied to the lower end of a vertical tubular reactor discharging at its upper end into primary cyclones which separate most of the catalyst from the reacted hydrocarbon vapors. Those vaporous reaction products then discharge into an enlarged zone before passing through secondary cyclone separators for removal of minor amounts of catalyst which remain suspended in the vapor products. Ideally, the conversion should terminate immediately at the top of the riser in order that there shall be no further conversion of the desired naphtha product to light gases. The disengaged catalyst contains, in addition to the non-volatile coke, a significant amount of volatilizable hydrocarbons which can become product if recovered, but constitute further fuel load on the regenerator if not removed from the spent catalyst. It is customary to pass disengaged catalyst, including that separated in the cyclones through a stripping zone in which it passes in counter-current contact with steam to volatilize hydrocarbons and strip them from the catalyst. Stripping steam with stripped hydrocarbons pass from the reactor with the disengaged vapor product to fractionation and recovery of the several products of the reaction. As would be expected, the adsorbed hydrocarbons, including naphtha components are subject to further conversion until finally removed by action of the stripping steam.
A recent development in catalytic cracking is the "stripper cyclone" for riser reactors as described in U.S. Pat. No. 4,043,899, dated Aug. 30, 1977, the entire disclosure of which is incorporated herein by this reference. Using the stripper cyclone technique, the suspension of catalyst in reaction product vapor is discharged from a riser into a cyclone having a spiral steam stripper section integral therewith. By this technique volatile hydrocarbons are removed from contact with the catalyst promptly after leaving the riser. This is shown to provide greater selectivity for gasoline at the same conversion level since naphtha components are subject to a lower possibility of further conversion.